Coating magnesium and magnesium base alloys



Patented Feb. 7, 1950 COATING MAGNESIUM AND MAGNESIUM BASE ALLOYS Norman H. Simpson and Paul R. Cutter, Fort Worth, Tex., Aircraft Corp oration,

assignors to Consolidated Vultee San Diego, Calif, a corporation of Delaware No Drawing. Application July 12, 1944, Serial No. 544,640

6 Claims.

The invention relates to protectively coating magnesium or magnesium base alloys.

Magnesium alloys, because of their strength combined with lightness, provide definite advantages when used in structures in aircraft and other industries. However, structures madeof magnesium alloys must be protected againstfatmospheric corrosion, electrolytic corrosion when in contact with dissimilar metals, mechanical injury, and from catching fire in air. Chemical and electro-chemical processes heretofore known for coating such metals have been found unsatisfactory and disadvantageous, particularly in mass production.

Magnesium alloys are relatively soft metals, and, therefore, must not only have resistance to corrosion, but possess a finish which is highly resistant to abrasion.

Heretofore, different current densities were required for each of the various alloys, according to their compositions. For this reason, the simultaneous or mass treatment of diiferent alloys could not be made due to excessive gassing in the electrolyte at some of the parts, which caused either pitting or burning of the surfaces of such parts. Also, the anodic coatings thus obtained were not compatible with paints.

One object of the invention is to provide a method of electro-chemically uniformly coating magnesium or magnesium alloys or articles in mass production.

Another object of the invention is to provide an improved method for coating magnesium and magnesium alloys and articles formed thereof, of difierent compositions, which produces a uniform finish highly resistant to corrosion and abrasion.

Another object of the invention is to provide a method of electro-chemically coating magnesium and magnesium alloys and articles formed thereof, to produce a highly smooth finish, and which is highly compatible with paint.

A still further object of the invention is to provide a method for producing a strong and tough coating on magnesium and magnesium alloys and articles formed thereof.

In practice, it has been found that these objects can be attained or contributed to by the use of a depolarizing agent in the electrolyte which functions to decrease or control the formation of gases on the material being coated, so that substantially uniform oxidation results on the surfaces of the metal being coated, even though there are variations in the compositions with consequent variations in the current den- 'Phenol--.09 to .15%

sity at said surfaces. It has been found that the OH ion concentration determines the potential at which the oxygen evolution begins. The higher the concentration of sodium hydroxide, NaOH, the lower the potential at which oxygen gas will be formed rather than oxidation of the magnesium anode that is, the over voltage for oxygen is lowered with increased OH. In mass production it has been found that a depolarizing reagent prevents excessive formation of gas which, if permitted, produces an inferior anodic film, particularly when the parts are of different types of magnesium alloys.

Another object of the invention is to provide a method of coating magnesium, magnesium alloys and articles formed thereof, which prevents excessive formation of oxygen gases, to improve the protective film.

Another object of the invention is to provide articles formed of magnesium and its alloys with an improved protective coating.

Other objects of the invention will appear from the detailed description.

The invention consists in the several novel features hereinafter set forth and. more particularly defined by claims at the conclusion hereof.

According to the present invention, the magnesium or magnesium alloys or parts, such, for example, as sheets, castings, forgings or tubing, formed thereof are immersed in an alkaline aqueous solution and then rinsed in hot water or subjected to a vapor degrease treatment or trichlorethylene for about 5 minutes, without rinsing, to sufficiently degrease all of the surface of the metal or articles, preparatory to the anodizing treatment.

Next, the articles of magnesium or magnesium alloys are immersed in an electrolyte in a suitable anodizing tank. The preferred electrolyte for carrying out the invention consists of an aqueous solution which comprises the following ingredients:

Sodium hydroxide-21 to 25% by weight,

by weight, and

Sodium silicates-2 to 3% by volume.

The temperature of the electrolyte is maintained from to F.

Direct current, approximately 4 amperes per sq. ft. is passed through the electrolyte with the articles forming the cathode, for about 5 minutes, which electrolytically cleans all the surfaces thereon.

Next, alternating current, preferably of 60 cycles and 15 to 25 amperes per sq. ft., is passed 3 through the electrolyte for a period ranging from 25 to 35 minutes.

This anodizing treatment will produce an anodic coating comprising, about 67% of magnesium hydroxides; amount 6 to 8 of magnesium silicates; about 2 to 3% of magnesium organic compounds (magnesium phenolate) and about 20 to 22% of moisture or water of crystal-- lization. I II I I I II The phenol in the electrolyte exemplifies an organic additive of the class of hydroxy organic compounds which forms polar groups when added to the caustic solutions and at the same time enters into the anodic coating formed, and reduces the porosity of the coating and renders the surfaces smooth, decorative and highly compatible with paint. The phenol is a depolarizing agent and serves to maintain an active magnesium anode at a potential low enough that passivity of the anode and excessive oxygen gas evolution does not occur. By preventing excessive oxygen gas evolution phenol prevents pitting of the surfaces being treated and causes more uniform anodic films to be formedthereon to provide the desired smooth finish. With a cutting down in oxygen gas evolution the electrical energy expended in the anodizing treatment will be less so that the utilization of phenol in addition to providing a better surface coating results in a saving of energy. I

In the mass or commercial production of magnesium parts the articles of I a batch simultaneously treated in an anodizing tank are not, in many instances, of the same compositions nor do they always have the same metallurgical history; certain parts may, for example, be formed of the common magnesium base alloy comprised of magnesium, aluminum, zinc and manganese, and other parts might be formed of a second popular magnesium base alloy comprised of magnesium and manganese. Parts having differences in composition and parts having a different metallurgical history, i. e., differences in manner of heat treatment, forming hardness, etc, will have different electro-chemical characteristics and when such parts having either different compositions or different metallurgical histories are simultaneously treated it has been found that many of the parts would not anodize, but would produce large quantities of oxygen. The depolarizing action of the additive in the electrolyte causes uniform distribution of current density to the surfaces being coated and results in producing a uniform coating having a smooth and dense surface on all of the articles simultaneously anddized. This smoothness is advantageous when coating aerodynamic articles or structures. As a result, the invention provides a method which is advantageous in mass production, particularly where it is necessary or desirable to simultaneously anodize a large batch of articles formed of magnesium base alloys of difi'ere'n't composi' tions as of different metallurgicalhistories. I

The articles are next rinsed in boiling water for about minutes to removethe electrolyte salts. Next, the coated articles are rinsed for about 5 minutes in a chromic acid solution (pHA-Ei) at a temperature ranging from 120 to 140 F. which neutralizes the residual caustic on the coating after the rinse in boiling water. I I I Next, the articles are dried with hot air for about 5 minutes. I

The coated articles, after havin'g been dried, may then be dipped for about 10 minutesin a bath of zinc chromate primer (consisting of two "scribed, it is to be understood silicate parts of thinner and 1 part of primer) to form an organic seal on the anodized coating.

The finish produced, when phenol is used in the electrolyte, is green in color and is naturally decorative without the addition of dyes. If desired, however, the color may be varied by the addition of dyes to the electrolyte.

In some instances it is desirable to paint the finished structure or parts. The electrolyte with phenol, which produces magnesium phenolate, results in a coating having a finish which has a high compatibility with paint. Heretofore it has been deemed desirable in aircraft used in warfare to apply at least several coats of paint for additionally protecting the surfaces.

A single coat of paint on anodized coatings containing magnesium phenolate has been found sufficient for that purpose.

While the desired results set forth are attained by the preferred electrolyte hereinbefore dethat the invention in its broadest aspect is not limited to the preferred electrolyte and procedure hereinbefore described. Any organic compound which forms 'polar groups upon addition to the caustic solution may be used, of which the following are examples: Hydroxy organic compounds .05% to "10.00% by weight, such as: glycerol, calcium gluconate, dextrose, cresol, phenolsulfonic acid, which when used in the'electrolyte as an anode depolarizer produce a smooth finish having characteristics similar to those produced when phenol is used, which may also be used within said range.

Direct current, preferably 3 to 5 amperes per sq. 'ft. from 25 to 30 minutes during the anodizing treatment, may be used in lieu of the alternating current heretofore described when the alloys are not of the high manganese type of magnesium alloys. When direct current is used, the electric connections for the energy are made so that the metal articles Will function as the anode.

The invention exemplifies a method of coating articles formed of magnesium or magnesium base alloys which is highly advantageous in mass production, and which produces a coating which is highly resistant to abrasion and corrosion.

The invention also exemplifies articles formed of magnesium and magnesium alloys with a coating which contains magnesium organic compounds which produces a surface which is smooth, can be sealed, and has a high compatibility with paint when that is desired.

The invention is not to be understood as restricted to the details set forth, since these may be modified within the scope of the appended claims without departing from the spirit and scope of the invention.

I Having thus described the invention, what we claim as new and desire to secure by Letters Patent is:

1. That improvement in forming a protective anodic coating on magnesium and magnesium base alloys which comprises, electrolytically treating them in a heated alkaline aqueous solution consisting of sodium hydroxide 21 to 25% by weight, phenol .09 to .15% by weight, and 'so'diumsilicate 2 to 3% by volume.

That improvement in forming a protective a'nodic coating on magnesium and magnesium base alloys which comprises, electrolytically treating them in an alkaline aqueous soiution consisting of sodium hydroxide 21 to 25% by weight, phenol, .09 to 15% by weight, and sodium to 3% by volume at a temperature ranging from to Fahr.

3. Magnesium and magnesium base alloys prov ded with an anodic coating consisting of magnesium hydroxide, substantially 67% by weight, magnesium silicate, substantially 6 to 8% by weight, and magnesium phenolate, substantially 2 to 3% by weight and the remainder water of crystallization.

4. A process for producing protective anodic coatings simultaneously on articles of difierent magnesium compositions, comprising electrolytically treating said articles in a heated aqueous electrolytic bath containing sodium silicate and sodium hydroxide, the sodium hydroxide being in the amount of 21 to 25% by weight and the sodium silicate being in the amount of 2 to 3% by volume, and to which bath is added .09 to .15% by Weight of phenol which forms polar groups upon addition to said bath and retards the formation of gases to eifect substantially uniform oxidation on the surfaces of the articles being coated and which enters into the anodic coating formed.

5. That improvement in forming a protective anodic coating on magnesium and magnesium base alloys, which comprises, electrolytically treating them in an aqueous electrolytic bath containing sodium silicate and an alkaline base, the quantity of sodium silicate used being 2 to 3 by volume and that of the alkaline base being 21 to 25% by weight, said bath also containing an additive of substantially .05 to 10.00% by weight of a hydroxy organic compound taken from the group which consists of glycerol, dextrose, cresol, phenol and phenolsulfonic acid, which forms polar groups and retards the formation of gases 1 to efiect substantially uniform oxidation on the surfaces of the metal being coated to produce a uniform coating, said additive becoming a part of the coating formed.

6. That improvement in forming a protective anodic coating on magnesium and magnesium base alloys, which comprises, electrolytically treating them in an aqueous electrolytic bath containing sodium silicate and sodium hydroxide, the quantity of sodium silicate used being 2 to 3% by volume and the sodium hydroxide being 21 to 25% by weight, said bath also containing an additive of substantially .05 to 10.00% by weight of hydroxy organic compound taken from the group which consists of glycerol, dextrose, cresol, phenol and phenolsulfonic acid, which forms polar groups and retards the formation of gases to effect substantially uniform oxidation on the surfaces of the metal being coated to produce a uniform coating, said additive becoming a part of the coating formed.

NORMAN H. SIMPSON.

PAUL R. CUTTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of The Electrochemical Society," vol. 81 (1942), page 353.

Ser. No. 268,694, Berthier (A. P. 0.), published June 8, 1943. 

5. THAT IMPROVEMENT IN FORMING A PROTECTIVE ANODIC COATING ON MAGNESIUM AND MAGNESIUM BASE ALLOYS, WHICH COMPRISES, ELECTROLYTICALLY TREATING THEM IN AN AQUEOUS ELECTROLYTIC BATH CONTAINING SODIUM SILICATE AND AN ALKALINE BASE, THE QUANTITY OF SODIUM SILICATE USED BEING 2 TO 3% BY VOLUME AND THAT OF THE ALKALINE BASE BEING 21 TO 25% BY WEIGHT, SAID BATH ALSO CONTAINING AN ADDITIVE OF SUBSTANTIALLY .05 TO 10.00% BY WEIGHT OF A HYDROXY ORGANIC COMPOUND TAKEN FROM THE GROUP WHICH CONSISTS OF GLYCEROL, DEXTROSE, CRESOL, PHENOL AND PHENOLSULFONIC ACID, WHICH FORMS POLAR GROUPS AND RETARDS THE FORMATION OF GASES TO EFFECT SUBSTANTIALLY UNIFORM OXIDATION ON THE SURFACES OF THE METAL BEING COATED TO PRODUCE A UNIFORM COATING, SAID ADDITIVE BECOMING A PART OF THE COATING FORMED. 